96 REPORT UNITED STATES GEOLOGICAL SURVEY. 



the waters they contain aid in shaping and enlarging the latter. Unless 

 the strata are inclined this feature is not so noticeable, because then the 

 valleys are cut in more regularly, and do not follow the strata of the 

 shales, but iu preference the dip, which, though scarcely perceptible, 

 generally exists. Vegetation partly preserves, partly destroys, the detail- 

 features of regions where shales predominate. In the former instance 

 it is the small plants that, by their matted roots, protect the easily-dis- 

 integrating material from erosion ; in the latter it is the large plants that, 

 loosening the shales, render them more readily transportable. As a rule, 

 however, shales are eroded altogether too rapidly to admitof any consider- 

 able accumulation of small vegetation on the steep slopes they [)resent, 

 so that they are unprotected against atmospheric and other eroding 

 agents. Owing to the steepness of these slopes, but a small amount of 

 the detritus remains upon them, but rolls or slides down and forms 

 a yielding talus at the base of the bluff or cliif which has furnished 

 the material requisite. 



Land-slides, mostly of small extent, occur quite frequently in shales 

 and in shales interstratified with sandstones. They are due mainly to 

 the existence of " water-strata." As such I designate strata or layers of a 

 very tough, impenetrable shale. Water precipitated upon the surface 

 percolates through the fissures, &c., of the various beds, until it reaches 

 a water-stratum. Owing to the physical constitution of the latter, it 

 is prevented from descending farther, excepting at such places where a 

 break may occur in the stratum. The result is that small rivulets of 

 water are formed between the water-stratum and the succeeding bed 

 above it. An inclination of the beds will direct the course of the water. 

 Wherever opportunity may be afforded, the latter will continue its 

 downward course, and wherever the conditions are favorable it will 

 emerge from the side of a bluff or in a valley, in the form of springs. 

 Should the inclination be sufficiently steep, the comparative separation 

 of two strata, by this accumulation of water, will often result in a down- 

 ward movement of the superincumbent beds, facilitated by the smooth 

 surface which the water produces. It is thus that land-slides are so 

 frequently formed in regions where shales are abundant. 



Although the water-strata themselves consist, almost invariably, of 

 shales, they are not confined to any special formation, but are found in 

 all or nearly all sedimentary groups. In sandstone and limestone strata 

 they occur, producing the same results as elsewhere. In a region that 

 is but si)aringly supplied with flowing water the recognition of their 

 existence is of great importance, furnishing, as it does, a clew to the 

 localities in which springs may be found. On the faces of bluffs the 

 water-strata are generally indicated by moist horizons, by efflorescence 

 of epigene minerals or by stains produced by hydrated oxygen com- 

 pounds of iron and manganese. Where the relative position ot a 

 stratum of this kind has been established, it will materially aid in the 

 discovery of existing springs, or in the selection of the most favorable 

 localities for wells. Its horizontal extension, of course, is not without 

 breaks and dry portions ; but, as a rule, the existence of water may 

 be depended upon at such places, which would afford the greatest facil- 

 ity for the egress of water at the relative niveau of the water-stratum. 



Deposited with the aid of chemical action. — To this group belong essen- 

 tially the limestones and dolomites. They differ in general surface-fea- 

 tures from the preceding series. Instead of a purely mechanical erosion, 

 we have here mainly a decomposition, or chemical alteration, and a subse- 

 quent removal of such altered portions. General atmosi)heric agents, 

 more particularly water, have the greatest effect, chemically, upon the 



